Gas sensor arrangements of this type are known for detecting a wide range of analytes, for example methane or carbon dioxide. Gas sensors, as described, for example, in EP 0616207 A2, WO 00/55603 A1 or in DE 19925196 C2, are based on the characteristic of many polyatomic gases to absorb radiation, in particular within the infrared wavelength range. In this process, this absorption appears in a wavelength which is characteristic of the relevant gas, for example at 4.24 μm for CO2. Thus, using infrared gas sensors of this type, it is possible to establish the presence of a gas component and/or the concentration of this gas component. Known gas sensors comprise a radiation source, an absorption path, i.e. a measuring chamber and a radiation detector. The radiation intensity measured by the radiation detector is a measurement of the concentration of the absorbent gas.
A broadband radiation source, usually a lamp, is generally used and the wavelength which is of interest is selected via an interference filter or grid. This type of radiation generation is also known as a non-dispersive method and, in the case of infrared-CO2 analysis, is termed the non-dispersive infrared (NDIR) method.
The detection of carbon dioxide is currently becoming increasingly significant in the automotive field. On the one hand, this is due to the fact that the CO2 content of the air inside motor vehicles is monitored in order to increase the energy efficiency for heating and air-conditioning, in order to induce a supply of fresh air via a corresponding fan flap control only when required, i.e. when the concentration of CO2 increases. On the other hand, modem air-conditioning systems are based on CO2 as a coolant. Thus, CO2 gas sensors may perform a monitoring function in connection with CO2 escaping in the event of possible defects.
Particularly in the automotive field, sensors of this type must, however, meet the highest requirements in terms of robustness, reliability and compactness, and long-term stability is required for many years. In this case, the emission of the infrared radiation source must remain stable over the entire service life, or must at least be monitored. However, with the requisite service life of a minimum of ten years and the currently conventional measuring rates of two seconds per measurement, the known IR radiation sources age too intensely to observe the specifications which have to be imposed on an NDIR gas sensor of this type.
Until now, two fundamental approaches have been known to counter this problem. Firstly, it is known to provide at least two beam paths with an infrared radiation source and two detectors, one of the detectors measuring the desired gas and the other measuring the brightness of the lamp with another wavelength. The change in the brightness of the lamp which is detected may be factored into a correction calculation using the second detector.
Another known solution, as described, for example, in DE 19925196 C2, uses at least two beam paths with two infrared sources and only one detector. The first lamp measures at the necessary measuring rate, while the second lamp is used only comparatively rarely for carrying out a comparative measurement. This solution assumes that the ageing of the second lamp is to be disregarded due to the intermittent switching-on.
These known solutions, however, suffer from the problem that on the one hand they are relatively complex and, on the other hand, to assess the lamp radiation they always require the detector signal, which is encumbered with the errors resulting from the long-term drift of the detectors and the parameter fluctuations occurring along the entire measuring path. Furthermore, in the event of an intermittent operation of a reference lamp, the ageing of said lamp can also no longer be disregarded with a service life in the region of ten years.